North Korea's military has raised its alert posture to its highest level in response to an annual joint military drill between Seoul and Washington, a government source here said Friday.

The North ordered its military on the highest special alert early this week as the allies kicked off their annual military exercise on Monday aimed at countering Pyongyang's potential aggression, according to the source.

This year's alert level was one notch higher than that issued during last year's joint military drill, indicating that North Korea is highly sensitive to the drill.

North Korea has long denounced the military drill as a rehearsal for invasion, but Seoul and Washington said that the exercise is defensive in nature.

North Korea threatened Monday to make a "preemptive nuclear strike" on South Korea and the U.S. in response to the allies' exercise which involves some 75,000 troops including about 25,000 from the U.S. side.

The source said that North Korea is also intensifying its military drills at front-line areas.

North Korea fired off a submarine-launched ballistic missile on Wednesday in an apparent protest against the Seoul-Washington military drill.

The North's leader Kim Jong-un hailed the latest missile launch as the "greatest success," claiming that his country has full capability to carry out nuclear attacks.

The missile flew about 500 kilometers toward Japan, marking the longest flight by such a missile and raising concerns about the technical advances in the North's missile program. (Yonhap)